Am I wrong to not spend more time there?

She doesn't want to lose weight she thinks the weight she is is normal for her but I'm sure it would help her mobility.

Thank you, I have had therapy to help with the guilt but even the therapist said it won't change.
For the past 3 years I've been going every 2 weeks but it's too hard. This month she has one of my children visiting for a weekend and I'm spending a weekend at home. I told her I'm going away for a weekend next week and she cried and said what about me? Another weekend by myself etc.
She has someone goes in to do her housework twice each week and a gardener one a week so chats to them.
My sibling lives in the next town to her and pops in for an hour twice each week.

Could you find someone you could pay to take up it mum out ?
But agree you cannot possibly commit to,more than once a month at that distance and nor should you feel obliged to. Is it impossible for your mum to lose some weight ?

No, I don't think you are being unreasonable. Your mother expects too much. You have your job and family to consider.
Maybe a compromise; possibly one weekend per month with her? It's natural that you feel guilty, divided loyalties do cause problems with guilt, but it is just impossible to do as your mother wants without upsetting your own life too much.